Combustible cartridge case and method of making same



May 2, 1961 F. BEM. Emp 2,982 2111 COMBUSTIBLE CARTRIDGE CASE AND METHOD OF' MAKING SAME Filed April :29.y 195s HNHYDROUS /V/TR/C HC/'D BY yf .WM/ Jj. MA a# or. a. 61W

Patented May 2, 1961 COMBUSTIBLE CARTRIDGE CASE AND METHOD OF MAKING SAME Keith F. Beal, Chicago Heights, Ill., and Walter D. Schmidt, Emerson, NJ., assignors to the United States of America as represented by the Secretary of the Army Filed Apr. 29, 1958, Ser. No. 732,781

Claims. (Cl. 102-43) The invention described herein may be manufactured and used by or for the Government for governmental purposes Without the payment to me of any royalties thereon.

The present invention relates to combustible cartridge cases and to a method of making same. More particularly the invention is concerned with laminated cellulosic cartridge cases which possess the required physical strength and which Will burn completely when red in conventional weapons.

The advantages of combustible cartridge cases will be apparent to those skilled in the art. The elimination of the conventional metal cases results in a considerable saving in weight; the problem of disposal is eliminated and there is a saving of metals which might become critically short in supply during times of emergency. Furthermore the tiring rates of weapons in which combustible cartridge cases are used may be increased substantially due to elimination of the ejection operation which would also result in a saving of Weight and space at the rear of the weapon. An additional advantage of combustible cartridge cases may involve the possible reduction in gun erosion. Prior methods and materials used for the manufacture of combustible cartridge cases have had the disadvantage that it has been diflcult to maintain the stability, strength and ballistic properties of the combustible material and further it has been diflicult to devise means for properly attaching primers and projectiles to the case.

Accordingly, a primary object of the present invention is the provision of a combustible cartridge case having the necessary strength characteristics and which Will burn completely when iii-ed in conventional weapons.

It is a further object of the invention to provide a nitrated cellulosic material suitable for use in the manufacture of combustible cartridge cases having superior strength, stability and ballistic properties.

A further object of the invention is the provision of a method for nitrating cellulosic material with concentrated nitric acid.

Other objects and advantages of the invention will become apparent to those skilled in the art from a study of the accompanying disclosure and drawings in which:

Fig. 1 illustrates the activity of commercial anhydrous nitric acid on cellulosic fabrics at various temperatures.

Fig. 2 illustrates the effect of acid concentration and temperature on the time required to reach a maximum degree of nitration.

Fig. 3 illustrates graphically the maximum degree of nitration attainable with various concentrations of nitric acid solutions.

Briefly the present invention contemplates the making of combustible cartridge cases by nitrating cellulosic fabrics with concentrated nitric acid, stabilizing the resulting cellulose nitrate and subsequently manufacturing cartridge cases by winding the nitrated fabric around a collapsible mandrel. A protective coating may be applied to the laminated case for the purpose of waterproong or, in some instances, as a means of controlling or retarding the burning rate of the combustible case.

In prior art methods of nitrating cellulose, it has been necessary to use a mixture of concentrated sulfuric acid and concentrated nitric acid, the sulfuric acid being the principal dehydrant in the process.

We have now found that superior nitration of cellulosic materials such as the non-woven rayon fabric known as Miracloth, cotton gauze or other natural or synthetic cellulosic materials, may be effected by treatment with commercially available, anhydrous nitric acid wherein the nitric acid acts as both the dehydrating and nitrating agent. Furthermore, the nitrating method of this invention possesses the additional advantage that it may be safely carried out at room temperatures whereas with prior art methods, a temperature of about 15 C. was considered to be the maximum permissible. Also, work done on the stabilization of cellulose nitrate indicates the obvious advantage that products obtained by this method are more easily stabilized than are cellulose nitrates obtained by conventional methods.

In each of the batch tests conducted in the development of this invention an excess of acid was used (approximately 100 ml. per gram of cellulose) and the acid was maintained at a uniform temperature by placing the reaction vessel in a water bath. The cellulose sample was immersed in the `acid bath for the specied time, after which the acid was rapidly removed from the vessel and sample, by vacuum or other suitable means. Ice Water was used to stop the reaction quickly. The nitrated material was then washed free of acid with several rinses of water and stabilized by methods to be discussed later.

The effects of reaction time, temperature and concentration on the rate and degree of nitration of cellulosic fabrics, and in particular cotton gauze, are illustrated in the acompanying drawings and tables.

Fig. `1 and Table No. 1 illustrate the reactivity at various temperatures of commercial anhydrous nitric acid which was found to have one percent each of titratable water and nitrogen dioxide. It will be observed from Fig. 1 that the maximum degree of nitration achieved with the anhydrous acid is relatively independent of the temperature at which the reaction proceeds. Irrespective of the temperature, the final product will have a nitrogen content of approximately 13.60 percent as compared to a theoretically fully nitrated cellulose having a nitrogen content of 14.14 percent. As shown in Fig. l, however, the rate of reaction is dependent upon the temperature. Thus at temperatures of 0 C. or higher 120 seconds or less is required to complete the reaction, whereas at a temperature of -25 C. approximately 400 seconds is required to attain the maximum nitrogen content of 13.60 percent.

TABLE 1 Ntration of M racloth (5 C.)

Percent Nitrogen N itration Period HNO; HN O3 HN O3 12. 87 10` 58 6. 05 12. l0. 52 6. 40 13. 07 ll. 72 6. 95 13. 20 ll. 58 7. 93 13. 3l 12. 38 8. 68

Fig. 2 illustrates the eiect of increased dilution on the reactivity of the acid at various temperatures. As will be seen, for any given concentration of acid, the maximum degree of nitration is again independent of temperature. However, two effects are to be recognized, namely that both the rate of reaction and the maximum obtainable nitrogen content decrease rapidly with decreasing acid concentration. Thus, at C., using anhydrous acid (Fig. 1) the reaction proceeded to completion in approximately 120 seconds. At the same ternperature, however, with 95 percent and 91 percent acid, nearly 300 and 4000 seconds respectively were required.

Fig. 3, in which the maximum obtainable nitrogen content is plotted against the Water concentration of the acid, illustrates the dependency of the degree of nitration upon the acid concentration at any given temperature.

As seen from the figures, a nitrated cellulose having any desired nitrogen content may be obtained by proper control of the time, temperature and concentration of the acid during the reaction.

The nitration method of this invention may be carried out successfully in either a batch-Wise or continuous process, and experiments conducted on the nitration of a continuous strip of cellulosic material at room temperature have been productive of cellulose nitrate having excellent characteristics and a nitrogen content of approximately 13.53 percent. Cellulose fabric materials have been found to be rather fragile, momentarily, at the time when they iirst enter the acid bath. At the same time a considerable dimensional shrinkage of the fabric occurs. Therefore, in nitrating strip material by a continuous process, it has been found desirable to support the material between two aluminum screen endless belts during its passage through the nitrating bath. After leaving the bath, excess acid solution should be squeezed out of the fabric which then passes directly into a series of wash or stabilizing baths. Table No. 2, wherein are tabulated the results of the nitration of a 126 foot continuous strip of cotton gauze, illustrates the degree of nitration achieved along the length of the strip. In this test, no attempt was made to maintain the acid at any definite concentration and therefore the degree of nitration is seen to decrease along the length of the strip as the nitric acid is depleted. During the experiment the bath temperature ranged from 65 F. at the start to 70 F. at the completion of the run. No acid was added and a speed of one foot per minute was maintained throughout the run. Stabilization of the product was achieved by five, one-half hour soakings in distilled water followed by a six hour boiling in distilled water.

TABLE 2 Nitraton of oven-dried 36 x 44 mesh cotton gauze Nitrogen Acid Stability 1 Sample (Ft. From Start) Content Strength (Min. to

(Percent) (Percent Fail) HNOa) l Hercules stability test.

1 minute at 30 degrees centigrade and in about 2 minutes at 0 degrees centigrade While with 91 percent acid about 10 minutes at 30 degrees centigrade and approximately 66.7 minutes at 0 degrees centigrade are required for the nitrogen content to reach a maximum. Control of Water concentration, reaction time and temperature provide a means of producing cellulose nitrate with any specified degree of nitration. This ability to fix the nitrogen content is considered an important expedient in controlling the ballistic properties of the cellulose nitrate.

The tensile strengths of nitrated Miracloth and cotton gauze fabrics as compared to untreated samples are illustrated by Table No. 3 wherein each value given represents the average result of tests made on ten samples.

TABLE 3 Tensile strengths of nitrated fabrics (UNLAMINATED-SINGLE SHEETS) 1 All samples i x 4".

Although the materials used in the development of the process of this invention were principally thin fabrics such as cotton gauze and Miracloth, we have found it possible to nitrate much heavier cellulose stock to a nitrogen content of about 13.5 percent in as little as 30 seconds. For example, viscose rayon fabrics having a thickness about one and one-half times the thickness of Miracloth and chemical cotton sheet stock made from cottonseed hull shavings pulp which is about ten times as thick as Miracloth, have been nitrated with beneficial results.

Table No. 4 illustrates the degree of nitration of such heavier fabrics. The advantages of the successful nitration of heavier gage fabric include the simplification of problems concerning fabrication into cartridge cases since fewer laminations are required.

TABLE N O. 4

Nz'traton of cellulose sheets in anhydrous nitric acid at 15 C.

HEAVY GAGE NON-WOVEN FABRICS Time of nitration: Percent nitrogen in product From the results obtained in the nitration of all types of fabrics both batchwise and continuously, it is evident that the concentration of the nitric acid must be kept quite high, preferably above 98 percent. Considerable speedup of nitration was obtained by raising the reaction temperature from 5 C. to 15 C. For example, with anhydrous acid at 5 C., a product containing 13.5 percent nitrogen required an average reaction time of 120 seconds as compared with an average of 45 seconds at 15 C.

The nitrated products of this invention may be satisfactorily stabilized either by treatment with aqueous sodium carbonate solutions or by thoroughly washing with boiling water. Preferably, stabilization is accomplished by treatment with boiling solutions containing approximately 1/2 percent by weight of sodium carbonate for a period of about five minutes. However, where the use of heat is undesirable an equivalent degree of stabilization may be achieved by treatment with the sodium carbonate solution at ambient temperatures for periods of from three to four hours although longer periods may prove beneficial under special conditions. Without the use of sodium carbonate solution as a neutralizing agent, comparable results are attainable only by boiling the material in water for at least four hours.

Any of a variety of convenient methods may be utilized for the fabrication of cartridge cases from the nitrated cellulose. Briefly, and in general, the nitrated cellulosic fabric is laminated by winding around a collapsible mandrel which is rotated under pressure against a rotating, internally heated, stainless steel cylinder roll. After completion of the winding the hot roll is stopped but the rotation of the cartridge case on the mandrel is continued for a short period. The use of a solvent to moisten the case during the laminating operation, has been found desirable to improve the strength of the lamination bond and the ultimate smoothness of the outer surface of the case. Apparently, the solvent acts to soften or dissolve nitrocellulose present on the surfaces of the bers and upon subsequent evaporation of the solvent, the fibers as well as the various layers of fabric are adhered together by a thin lrn of nitrocellulose. For this purpose any solvent for nitrocellulose may be employed, among them being diisobutyl ketone, ethyl acetate, butyl acetate, methyl ethyl ketone, cyclohexanone, nitrotoluene, nitrobenzene, solutions of nitrocellulose in acetone or ethyl acetate and solutions containing 5-10 percent diacetin in ethylacetate or methylethyl ketone.

In a preferred method of fabrication of nitrocellulose cases, particularly those of larger caliber, it has been found advantageous to form the case over a layer of aluminum screen wrapped about the collapsible mandrel. As the nitrated gauze is wrapped about the screencovered mandrel, itis brushed with an adhesive consisting of either a 1% solution of nitrocellulose or a 10% solution of ethyl cellulose in ethyl acetate. In this manner, after removal of the screen and case from the mandrel, the screen assists in maintaining the shape of the case until all moisture and solvent has been removed. Since in this method, both the inner and outer surfaces of the case are exposed to the air, evaporation of the solvents is not significantly obstructed. A vacuum drying arrangement may be employed as a means of increasing the rate of drying. Fabrication of the cartridge cases may be accomplished with or without the use of an organic solvent to aid in the lamination. Where no solvent is employed, the water-wet, nitrated fabric is simply wound about the mandrel and then oven dried at about 120 F. for a period of approximately twenty-four hours. If desired, the case may then be saturated by soaking or other means, in a solvent such as acetone. Cases made by these latter methods, however, require a great deal of smoothing with wet sand paper or the like while the mandrel and case are still mounted on a lathe. Preferably, an organic solvent such as ethyl acetate, isopropyl acetate, or butyl acetate is applied to the fabric as the water-wet material is being wound around the collapsible mandrel.

The method of nitration of this invention thus provides a superior nitrated cellulosic product having a nitrogen content of approximately,I 13.60 percent, which has distinct advantages in the manufacture of combustible cartridge cases. When tested in conventional weapons of various calibers, cartridge eases laminated from nitrated cellulose produced by the method of the present invention exhibit excellent strength, good impact sensitivity and favorable ballistic properteries. After tiring, no unburned residue was found in the gun or around the firing positions. The burning rate of these cartridge cases appear to be greater than that of the powder replaced in view of the fact that a higher peak pressure was indicated without a proportional increase in velocity.

In the foregoing description we have disclosed preferred embodiments of our invention. However, it is not intended that this invention be limited to the specific examples set forth above, as it will be apparent to those skilled in the art, that the proportions of the ingredients may be varied and a variety of equivalent substances may be employed without departing from the spirit of the invention or exceeding the scope of the appended claims.

Having thus described the invention, what is claimed as new, and desired to be secured by United States Letters Patent is:

1. The method of fabricating combustible cartridge cases capable of burning completely when red in conventional weapons comprising nitrating a cellulosic fabric material with substantially anhydrous nitric acid at room temperature and below and while still wet, laminating said material into the form of a cartridge case.

2. The method of forming a combustible cartridge case as recited in claim l wherein the nitrated fabric is treated with an organic solvent for nitrocellulose while being laminated.

3. The method of forming combustible cartridge cases capable of burning completely when red in conventional weapons comprising nitrating a cellulosic fabric with concentrated anhydrous nitric acid having an acid content of at least 98 percent, stabilizing the resulting nitrated fabric and laminating said fabric into the form of a cartridge case by winding about a layer of screen wrapped about a collapsible mandrel, and then removing the case from the screen and mandrel.

4. The method of claim 3 wherein the cartridge case is further saturated with an organic solvent for nitrocellulose, smoothed and then dried.

5. The method of claim 3 wherein said fabric is treated with an organic solvent for nitrocellulose during lamination.

References Cited in the le of this patent UNITED STATES PATENTS 33,393 Johnson Oct. 1, 1861 88,948 Ely Apr. 3, 1869 1,184,753 Keplinger May 30, 1916 2,564,695 Johnson et al Aug. 21, 1951 

1. THE METHOD OF FABRICATING COMBUSTIBLE CARTRIDGE CASES CAPABLE OF BURNING COMPLETELY WHEN FIRED IN CONVENTIONAL WEAPONS COMPRISING NITRATING A CELLULOSIC FABRIC MATERIAL WITH SUBSTANTIALLY ANHYDROUS NITRIC ACID AT ROOM TEMPERATURE AND BELOW AND WHILE STILL WET, LAMINATING SAID MATERIAL INTO THE FORM OF A CARTRIDGE CASE. 